There is a known technique for acquiring behavior information including an understeer state of a user's vehicle by determining whether the vehicle is in the understeer state based on a deviation between an actual motion state (actual yaw rate) of the vehicle and a motion state (model yaw rate) serving as a model of the vehicle (see, for example, Japanese Unexamined Patent Application Publication No. 2011-79395 (see paragraphs 0038 to 0039).